Systems that operate on sensitive data need to protect against the unauthorized access to, or disclosure or alteration of, such data by attackers. Attackers who gain access to cryptographic keys and other secrets could steal or tamper with the sensitive data, leading to severe consequences such as subversion of critical operations of the system through the introduction of unauthorized commands and the exposure of confidential or proprietary information. One compromised element may also be used to mount further attacks, endangering other elements of a system. More specifically, previous research has shown that an attacker can monitor a device's external characteristics such as operation timing, power consumption and/or electromagnetic radiation and use this additional information to extract the secret keys being used within the device. For example, as described by Kocher et al (see P. Kocher, J. Jaffe, B. Jun, “Differential Power Analysis,” Advances in Cryptology—Crypto 99 Proceedings, Lecture Notes In Computer Science Vol. 1666, Springer-Verlag, 1999), it is well known in the art that external monitoring of a device performing a sequence of cryptographic operations using the same set of keys with different data can result in the leakage of the key.
Because external monitoring attacks are typically passive and non-invasive, traditional tamper resistance defenses which are based on thwarting physical access or detecting improper usage are insufficient or impractical to provide protection against such attacks. For example, methods for managing secret keys using physically secure, well-shielded rooms are known in the background art. However, in many applications, requiring cryptographic systems to remain in physically isolated facilities is not feasible, given the environments in which they are expected to operate. In addition, such facilities are expensive to build and operate, and may still be imperfect in their ability to prevent small amounts of information from leaking to adversaries.
Of course, other methods are known in the background art that can mitigate the problem of information leakage from monitoring attacks without necessarily relying on physical shielding. These include methods for reducing the amount (or rate) of information leaking from transactions, modifying cryptographic algorithm implementations to randomize computational intermediates, and/or introducing noise in power consumption and operation timing.
For example, U.S. Pat. No. 6,539,092, entitled “Leak-Resistant Cryptographic Indexed Key Update,” provides methods for converting a shared master key and an index value (e.g., a counter) into a transaction key, where the derivation is protected against external monitoring attacks. Those methods work well in applications where the device(s) being protected against external monitoring attacks can contribute to the derivation of the transaction key. For example, the '092 patent describes how a smartcard can maintain an index counter which increments with each transaction, then use the index counter in the key derivation.
There are applications, however, where the participant(s) in a protocol should be protected against external monitoring attacks, but lack the ability to store sequence counters and updated keys, as described in the '092 patent. For example, consider the case where a device needs to regularly process the same input data, such as a device which contains a fixed and unchanging embedded key that is repeatedly used to decrypt ciphertexts in arbitrary order. Firmware encryption is an example of such an application; a microprocessor may be manufactured having an embedded key in fuses, and on every reboot the microprocessor needs to re-decrypt its firmware image loaded from an untrusted external flash. The firmware image may occasionally be updated, but the same ciphertext may also be decrypted repeatedly. Thus, both the application requirements and the physical manufacturing limitations (such as the inability to modify stored keys due to the use of one-time-programmable fuses to hold keys) can make it impractical for the device to limit the number of times the decryption key will be used. The firmware publisher could use the methods described in the '092 patent with a new index value each time a new encrypted firmware image is released, but the decrypting device cannot use a different index value on each reboot, since changing the index value to a value other than the one used by the encrypting device would result in an incorrect decryption. Thus, an attacker can potentially supply the decryption device with tampered data sets, then attempt to recover the secret key by monitoring external characteristics while the device processes (e.g., decrypts, etc.) these ciphertexts. Statistical side channel attacks, such as differential power analysis (DPA), can deduce a secret key from a set of measurements collected when a device uses the same key repeatedly to operate on different input values (such as the different firmware ciphertexts or tampered versions of the same firmware ciphertexts in the foregoing examples). Measurements from a single long message (e.g., comprising many block cipher inputs) or a collection of legitimate messages (such as multiple firmware versions) may also provide sufficient data for a side channel attack, even if ciphertext messages are not tampered.
Of course, in some situations where a device uses the same key for every transaction, the device could theoretically implement a lock-out (e.g., by self-destructing if a transaction or failure threshold is exceeded) to limit the number of transactions an adversary can observe. Lock-out mechanisms, however, introduce numerous practical problems, however, such as reliability concerns and the difficulties associated with storing a failure counter (e.g., many semiconductor manufacturing processes lack secure on-chip nonvolatile storage, and off-chip storage is difficult to secure).
In light of all the foregoing, a method that provides a verifiably secure way for devices to communicate and exchange data, with protection against external monitoring attacks and the ability for devices to reject non-genuine data, would be advantageous.